Perry's Index to the Aesopica

Fables exist in many versions; here is one version in English:

THE WOLF AND THE SHEPHERD

A wolf followed along after a flock of sheep without doing them any harm. At
first the shepherd kept his eye on the wolf as a potential enemy to the flock
and never let him out of his sight. But as the wolf continued to accompany the
shepherd and did not make any kind of attempt to raid the flock, the shepherd
eventually began to regard the wolf more as a guardian of the flock than as
a threat. Then, when the shepherd happened to have to go to town, he commended
the sheep to the wolf in his absence. The wolf seized his chance and attacked
the sheep, slaughtering most of the flock. When the shepherd came back and saw
that his flock had been utterly destroyed, he said, 'It serves me right! How
could I have ever trusted my sheep to a wolf?' The same is true of people: if you entrust your bank deposits to greedy
men, you are certain to get robbed.

Source:
Aesop's Fables. A new translation by Laura
Gibbs.
Oxford University Press (World's Classics): Oxford, 2002.
NOTE: New
cover, with new ISBN, published in 2008; contents of book unchanged.

You can find a compilation of Perry's index to the Aesopica in the gigantic appendix to his
edition of Babrius and Phaedrus for the Loeb Classical Library
(Harvard University Press: Cambridge, 1965). This book is an absolute must for anyone interested
in the Aesopic fable tradition. Invaluable.